1. Field of the Invention
The present invention relates to a power supply system for a camera which supplies electrical power to a light measuring circuit, a display circuit and others in a hold state which is held during a predetermined period of time before the camera turns into a standby state, and a camera system to control the operation of the light measuring circuit and the display circuit in the hold state.
2. Description of the Prior Art
Conventional cameras, having a display panel for displaying photographic information such as aperture value and shutter speed, turns into a standby state through a hold state held during a predetermined period of time. In the hold state of the conventional cameras a light measuring circuit, a display circuit and their control circuits are operating as usual. The purpose of the hold state is to facilitate a setting operation of the photographic information or confirmation of it after a shutter is released.
FIG. 2 shows an example of a power supply system for a camera of a conventional type. In FIG. 2, B is a battery as a power supply source; 1 is a power supply circuit; 2 is a booster circuit comprising a DC/DC converter etc. in the circuit 1; 3 is a control circuit comprising a microcomputer including a RAM which controls camera operation and also memorizes data such as photographing conditions (shutter speed, aperture value, etc.) in the RAM; 4 is a display circuit using a liquid crystal device; 5 is a motor driving circuit; 6 is a motor for driving a lens etc.; 7 is a switch to start the control operation of the microcomputer in the circuit 3.
An input terminal Vin of power supply circuit 1 is connected to a positive electrode of the battery B; an output terminal Vout in the power supply circuit 1 is connected to power supply input terminals Vcc1 and Vcc2 of the control circuit 3 and the display circuit 4 respectively, and also connected to an anode of a voltage regulating diode D4 through a resistor R2; the anode of the diode D4 is connected to a reference voltage terminal Vr of the display circuit 4 and the cathode is grounded; a power supply control terminal PWC of the control circuit 3 is connected to a control terminal Vc of the power supply circuit 1.
The positive electrode of the battery B is connected to a power supply input terminal Vcc3 of the motor driving circuit 5 and to an anode of a light emitting diode (LED) D3; a cathode of the diode D3 is connected to the display circuit 4 through a resistor R1; the motor driving circuit 5 is a load circuit which necessitates comparatively large current and therefore electrical power is directly supplied from the battery B.
The power supply circuit 1 is explained in the following: an input terminal IN of the booster circuit 2 is connected to the input terminal Vin of the power supply circuit 1, and an output terminal OUT of the booster circuit 2 is connected to the output terminal Vout of power supply circuit 1 through the diode D2; a control terminal C of the booster circuit 2 is connected to a control terminal Vc of the power supply circuit 1; the input terminal Vin of the power supply circuit 1 is connected to the anode of the diode D1, and the cathode of the diode D1 is connected to the output terminal Vout of the power supply circuit 1.
An OR circuit relating to the booster circuit 2 and the battery B is constituted by the diode D1 and the diode D2, and the circuit is so constituted that the output voltage of either of the two, the booster circuit 2 or the battery B, which has a higher voltage is output from the output terminal Vout.
The operation of the power supply circuit 1 is explained in the following: when the switch 7 is OFF and the control circuit 3 is not started, a control signal "HIGH" is output from the power supply control terminal PWC of the control circuit 3 to the power supply circuit 1, and the booster circuit 2 in the power supply circuit 1 is not in operating state; at this time, the voltage of the battery B is output to the output terminal Vout of the power supply circuit 1 through the diode D1 and it is supplied to the power input terminals Vcc1 and Vcc2 of the control circuit 3 and the display circuit 4, and the memory contents of the RAM in the control circuit 3 are held by the battery power; the voltage of the battery B is applied to the anode of the voltage regulating diode D4 and a reference voltage for driving the liquid crystal device is generated by the diode D4 and it is input to the reference voltage input terminal Vr of the display circuit 4; if the voltage of the battery B is lowered, it may cause the hold of the memory contents in the RAM impossible or the lowering the reference voltage, which degrades the contrast of display on the liquid crystal device.
The power for the motor driving circuit 5 and the light emitting diode D3 are supplied from the battery B independent of ON or OFF of the switch 7.
When the switch 7 is turned ON, a signal "LOW" is output from the power supply control terminal PWC and supplied to the control terminal Vc of the power supply circuit 1 to start the booster circuit 2 in the power supply circuit 1. At this time, the output voltage of the booster circuit 2, in which the battery voltage is boosted, is output to the output terminal Vout of the power supply circuit 1 through the diode D2 and is supplied to the respective power supply terminals Vcc1 and Vcc2 of the control circuit 3 and display circuit 4 and the microcomputer in the control circuit 3 starts the control operation. The output voltage of the booster circuit 2 is also applied to the anode of the voltage regulating diode D4 through the resistor R2 and the reference voltage for driving the liquid crystal device is generated by the diode D4.
When the sequence of the control operation of the microcomputer is advanced to the step in which the motor 6 for driving the lens etc. is driven, a large current flows in the motor driving circuit 5 and the voltage of the battery B is lowered; it causes to lower the applied voltage to the light emitting diode D3 and the brightness of the diode D3 is lowered. After that, when the control operation of the microcomputer is finished, the control signal "HIGH" is output from the power supply control terminal PWC of the control circuit 3 and supplied to the booster circuit 2 and then the operation of the circuit 2 is stopped, and the voltage of the battery is supplied to the control circuit 3, display circuit 4 and the voltage regulating diode D4 through the diode D1 in the power supply circuit 1.
Furthermore, since the control circuit 3, the display circuit 4 and the light measuring circuit (not shown) are supplied the electrical power as usual during the hold state, the electricity consumption during the hold state cannot be neglected.
As for the ways of decreasing power consumption, following examples are known to the public: a means wherein when an automatic exposure setting is held, the power supply to a measuring circuit is stopped (for example Japanese Patent laid-open No. 78429/85); a means wherein when a light measurement switch is kept ON for a certain time, the power supply to the light measuring circuit is cut off and the power supply is resumed with the ON of a shutter release switch (for example Japanese Patent laid-open No. 78430/85 and No. 78431/85); a means wherein power is supplied periodically in a fixed period during a light measuring switch is kept ON (Japanese Patent laid-open No. 140326/85).
In these known power supply system, matters after a light measurement switch is turned OFF are not taken into consideration. Therefore, there still remained a problem to be solved on the cut down of the power consumption to lengthen the time of the hold state.